National Transmission System

The system originated in the construction during 1962–3 of the {{convert|200|mi|km|adj=on}} high-pressure methane pipeline from Canvey Island (on the Essex coast) to Leeds.{{Cite book|title=A History of the British Gas Industry|url=https://archive.org/details/historyofbritish0000will|url-access=registration|last=Williams|first=Trevor I.|publisher=Oxford University Press|year=1981|pages=[https://archive.org/details/historyofbritish0000will/page/147 147]}} Imported liquefied natural gas (LNG) from Algeria was turned into gas at the Canvey terminal and supplied to the pipeline, providing eight of the twelve area gas boards with access to natural gas.{{Cite web|url=http://www.oldflames.org.uk/Plant%20Operations%20Department.pdf|title=Origins and growth of the British Gas Plant Operations Department |website=www.oldflames.org.uk}} The gas was initially used to manufacture town gas, either as a feedstock in gas reforming processes or to enrich lean gases such as that produced by the Lurgi coal gasification process.

The pipeline was {{convert|18|in|mm|adj=on}} in diameter and operated at 1,000 pounds per square inch (69 bar). The pipeline had {{convert|150|mi|km}} of spur lines, supplying gas to area boards.{{Cite book|title=The Modern Gas Industry|last=Wilson|first=D. Scott|publisher=Edward Arnold Ltd.|year=1969|pages=43}}

The Gas Council was responsible for this £10 million co-operative scheme and the construction details were a joint effort of the distribution engineers of the area boards.

LNG had first been imported to Canvey from Louisiana in February 1959, and piped to Romford gasworks as feedstock to a reforming plant.{{Cite book|title=Always under Pressure: A History of North Thames Gas since 1949|last=Falkus|first=Malcolm|publisher=Macmillan|year=1988|pages=63 & 74}}

Natural gas was discovered on the UK continental shelf in 1965 and production started in 1967.{{Cite book|title=Always under Pressure: A history of North Thames Gas since 1949|last=Falcus|first=Malcolm|publisher=Macmillan|year=1988|pages=76 & 108}} The development of offshore natural gas fields is shown in the following table. Shore terminals were built to receive, process, blend and distribute the gas.

With the assured availability of natural gas, a government white paper on fuel policyFuel Policy, Cmnd. 3438, HMSO, London (1967).{{Cite book|title=A History of the British Gas Industry|url=https://archive.org/details/historyofbritish0000will|url-access=registration|last=Williams|first=Trevor I.|publisher=Oxford University Press|year=1981|pages=[https://archive.org/details/historyofbritish0000will/page/210 210]–11}} in November 1967 proposed that natural gas should be immediately and more extensively exploited. The Gas Council and area boards began a ten-year programme to convert all users and appliances to operate on natural gas and consequently to discontinue the manufacture of town gas at local gasworks. In a pilot scheme, users on Canvey Island had been converted to natural gas in 1966.{{Cite book|title=A History of the British Gas Industry|url=https://archive.org/details/historyofbritish0000will|url-access=registration|last=Williams|first=Trevor I.|publisher=Oxford University Press|year=1981|pages=[https://archive.org/details/historyofbritish0000will/page/182 182]–9}}

To exploit the availability of natural gas and to provide for more widespread distribution, construction began of a major new transmission network which became the National Transmission System.John Ellis 'The Origins and Growth of the British Gas Plant Operations Department', 2014.

Gas from the West Sole field was first dispatched from the Easington terminal on the Yorkshire coast in July 1967, via Feeder No. 1 across the Humber to the East Midland Gas Board's gasworks at Killingholme. It was used to enrich low calorific value manufactured gas. Feeder No. 1 was extended to Totley near Sheffield where it connected to the 18-inch methane pipeline. UK natural gas first entered the NTS in July 1968.{{Cite book|title=Natural Gas|last=Tiratsoo|first=E.N.|publisher=Scientific Press Ltd.|year=1972|location=Beaconsfield|pages=216, 221, 222}}

Feeder lines from the North Sea gas terminals to the spine of the NTS were laid and brought into use as the shore terminals were constructed.{{Cite book|title=A History of the British Gas Industry|url=https://archive.org/details/historyofbritish0000will|url-access=registration|last=Williams|first=Trevor I.|publisher=Oxford University Press|year=1981|pages=[https://archive.org/details/historyofbritish0000will/page/177 177]–8}}{{Cite book|title=North Sea Heritage: the story of Britain's natural gas|last=Wilson|first=D Scott|publisher=British Gas|year=1974|pages=27}}{{Cite book|title=Natural Gas|last=Tiratsoo|first=E.N.|publisher=Scientific Press Ltd.|year=1972|location=Beaconsfield|pages=221–2}}

The No. 6 feeder runs via Pickering which received gas from a treatment plant for the onshore Lockton gas field.

North Sea gas first reached Scotland in Spring 1970 at Coldstream via an extension of the Leeds-Newcastle pipeline. This pipeline was then extended to Glenmavis near Coatbridge Lanarkshire (Feeder No. 12) where a natural gas liquefication plant was constructed.

A major set of pipelines were constructed in Scotland in preparation for arrival of gas from the Frigg gas field in 1977. From the St Fergus terminal in Scotland, two {{convert|36|in|mm|adj=on}} pipelines (Feeder No. 10 and No. 11) were laid via Bathgate to Partington and Bishop Auckland to connect to the NTS in England, a total pipeline length of {{convert|595|mi|km}}. These lines were commissioned in 1976 and cost £140 million. Initially they carried gas from southern England into Scotland until the Frigg field began production via St Fergus in September 1977. Compressor stations are provided at {{convert|40|mi|km}} intervals along the pipelines. A third 36-inch pipeline from St Fergus (Feeder No. 12) was completed in 1978, and a fourth {{convert|40|in|mm|adj=on}} pipeline (Feeder No. 13) in 1982.{{Cite book|title=Gas: Natural Energy|last=Cassidy|first=Richard|publisher=Frederick Muller Ltd|year=1979|location=London|pages=39–47}}

The NTS was extended from Leeds to Newcastle upon Tyne in early 1969. This line was extended to Coldstream in spring 1970 and then to Glenmavis, near Coatbridge, Lanarkshire.

The Wales Gas Board received natural gas supplies in 1969 through a 24-inch line from Churchover (Rugby) to Swansea via Wormington (an extension to Feeder No. 2). North Wales was also connected in 1969 via a 24-inch/18-inch pipeline from Audley Cheshire to Maelor near Wrexham (an extension to Feeder No. 4).

The South Western Gas Board received natural gas at the end of 1970 from a 24-inch/20-inch pipeline from Wormington to Exeter (Feeder No. 14).

A 30-inch/24-inch extension of Feeder No. 3 runs to the west of London via Slough to Mogador, Surrey, and was commissioned in 1970. An extension of Feeder No. 5 runs from Horndon-on-the Hill, crosses the Thames at Tilbury and runs via Shorne to connect to Mogador, completing the South London ring main which became operational in early 1972.

In addition to these distribution pipelines, in 1971 the area boards began to supply natural gas directly to major consumers. For example, a 24-inch 17 mile 'spine' pipeline was constructed to ICI Ltd at Billingham (designated as part of Feeder No. 6), and the West Midlands Gas Board laid six similar 'spine' mains into industrial districts of Birmingham and the Black Country.

Most of the NTS was built from the late 1960s to the early 1980s.{{Cite book|title=A History of the British Gas Industry|url=https://archive.org/details/historyofbritish0000will|url-access=registration|last=Williams|first=Trevor I.|publisher=Oxford University Press|year=1981|pages=[https://archive.org/details/historyofbritish0000will/page/225 225]–30}}

Later (post-1983) feeder mains not described above include:{{Cite web|url=http://www.google.com/maps/d/viewer?mid=1H3qFrNgSMKHjl7wIvEMwoWhbD6I&hl=en_US&ll=54.3268777141023%2C-0.61519577012632&z=6|title=NTS feeder mains}}

The NTS now comprises over 7,600 km of welded steel gas pipelines. The Canvey to Leeds line is no longer part of the NTS.

In addition to the Canvey Island liquefied natural gas (LNG) import terminal, further LNG storage sites were constructed from the late 1960s.{{Cite book|title=Natural Gas|last=Tiratsoo|first=E. N.|publisher=Scientific Press Ltd|year=1972|location=Beaconsfield|pages=159, 183, 224}} These were peak-shaving facilities used to support the NTS at times of high demand, and to ensure security of gas supplies at strategic locations. When demand was high, liquefied natural gas was pumped from storage tanks, heated in vapourisers to a gaseous state and delivered into the NTS. When demand was low, gas was withdrawn from the NTS and liquefied by cryogenic cooling to minus 162 °C to replenish the storage tanks.

In addition to LNG storage for peak-shaving, several sites had storage facilities for high pressure gas that could be released into, and pressurised from, the NTS. The following sites were operational by 1972.{{Cite book|title=Natural Gas|last=Tiratsoo|first=E.N.|publisher=Scientific Press Ltd|year=1972|location=Beaconsfield|pages=174}}

• Isle of Grain, Kent: six 'bullets', {{convert|12|ft|m|abbr=on}} diameter, {{convert|250|ft|m|abbr=on}} long, capacity 8 million cubic feet (226,000 m³) of gas, operating at up to 1,000 psi (69 bar).
• Beckton gasworks, East London: eight 'bullets', {{convert|13.5|ft|m|abbr=on}} diameter, {{convert|263|ft|m|abbr=on}} long, capacity 5 million cubic feet (142,000 m³) of gas, pressure cycle 350-100 psi (24–6.9 bar).
• South Western Gas Board, for Bristol and Cheltenham: eleven 'bullets', {{convert|13|ft|6|in|m|abbr=on}} diameter, {{convert|311|ft|8|in|m|abbr=on}} long, total capacity 13 million cubic feet (368,000 m³), pressure cycle 450-40 psi (31–2.76 bar).
• Biggin Hill, Kent: seventeen, {{convert|42|in|m|adj=on}} diameter buried pipes, {{convert|1,040|ft|m|abbr=on}} long, capacity 10 million cubic feet (283,000 m³), operating up to 1,000 psi (69 bar).

The NTS is the starting point for UK gas distribution. The pipeline system serving houses is not part of the NTS, but is part of the gas distribution network of local distribution zones; the two systems combine to form the UK's gas distribution network.

The two types of gas pipelines in the UK are: large diameter high-pressure (up to 85{{Cite web|url=http://nationalgrideducation.com/_assets/downloads/education_resources/ngrid_be-the-source_how-gas-produced.pdf|title=The domain nationalgrideducation.com is registered by NetNames|website=nationalgrideducation.com}} bar (1200 psi) and {{convert|1050|mm|in|frac=4|abbr=on}} diameter) pipelines – the type that the NTS uses – and smaller diameter lower pressure pipelines that connect to users who burn gas for heat. The wall thickness of the high-pressure pipelines is up to {{convert|0.625|in|mm}}.

Gas currently enters the NTS from a number of sources:

• Offshore oil and gas fields on the UK continental shelf. These deliver gas via five (formerly six) UK coastal gas terminals (five in England: CATS Teesside; Easington/Dimlington; Bacton; Rampside Barrow and the former Theddlethorpe terminal; and one in Scotland: St Fergus). Gas from the Liverpool Bay (Douglas) field formerly entered the NTS at Burton Point terminal in Cheshire; this terminal is now identified by National Grid as a NTS offtake to Connah's Quay power station.
• Onshore gas fields such as Saltfleetby, Lincolnshire (production was via the former Theddlethorpe terminal); and Wytch Farm, Dorset.
• Continental Europe. From Norway via the Langeled pipeline and the Easington terminal; from the Netherlands via the BBL pipeline; from Belgium via the Interconnector UK pipeline, both of the latter through Bacton gas terminal.
• Imported LNG. Gas is delivered from import terminals at the Isle of Grain and Milford Haven (South Hook and Dragon). The Canvey Island gas terminal ceased importing LNG in 1984.{{Cite web|url=http://www.canveyisland.org/page_id__888.aspx|title=Methane gas terminal Canvey Island}}
• Storage facilities. These include a mixture of salt cavity storage, onshore LNG storage sites, and formerly the depleted onshore gas field at Rough (via Easington terminal). The onshore storage facilities are listed below. The NTS was formerly supplied by the following decommissioned LNG sites: Ambergate, Derbyshire (closed 1985); Dynevor Arms, Merthyr Tydfil (closed 2009); Glenmavis, Lanarkshire (closed 2012); Partington, Greater Manchester (closed 2012); and Avonmouth, Bristol (closed April 2016).

The specification of gas transported within the NTS is typically within the following parameters (revised April 2023).{{Cite web|url=https://www.gasgovernance.co.uk/sites/default/files/ggf/2023-03/GSMR%20Review%20Implementation%20-%20Transmission%20WG%20-%206.4.23.pdf |title=GS(M)R Review: Gas Quality Specification Changes |website=www.gasgovernance.co.uk}}{{Cite web|url=http://www.legislation.gov.uk/uksi/1996/551/contents/made |title=Gas Safety (Management) Regulations, 1996 |website=www.legislation.gov.uk}}{{Cite web|url=http://www.shell.co.uk/promos/bacton/_jcr_content.stream/1451289032675/46fb43f9c78afb11b7670a46955bfa91129605083ae8bf73400ed6bfe833c428/bacton-asset-fact-sheets.pdf |title=Shell Bacton Infrastructure |website=www.shell.co.uk}}{{Cite web|url=http://www.perenco-uk.com/sites/perenco-uk.com/files/documents/Infrastructure_Access_Information_North.pdf |title=Perenco Bacton & Dimlington Infrastructure |website=www.perenco-uk.com}}

Parameters marked * are specified in the Gas Safety (Management) Regulations 1996. As amended by the Gas Safety (Management) Regulations 2023.Gas Safety (Management) Regulations 2023

The composition of natural gas in the NTS in 1979 was typically as shown below.{{Cite book|title=Gas: Natural Energy|last=Cassidy|first=Richard|publisher=Frederick Muller Limited|year=1979|location=London|pages=14}} The decline of supply from the UK continental shelf since 2000{{Cite web|url=https://www.nationalgas.com/insight-and-innovation/gas-ten-year-statement-gtys |title=2023 Gas Ten Year Statement |website=www.nationalgas.com|access-date=2023-12-28}} and the sharp increase in LNG imports during the 2021 natural gas supplier crisis have made the composition more variable, though still within the calorific value limits.

Gas storage is used to manage seasonal and short-term variations in the supply and demand of gas in the UK. Facilities include salt caverns and onshore LNG storage sites (see above). Since 1985, gas has been stored offshore in the depleted Rough gas field, which initially held nine days' supply and was the UK's largest store.{{Cite web|last=Vaughan|first=Adam|date=2017-06-20|title=Closure of UK's largest gas storage site ‘could mean volatile prices’|url=http://www.theguardian.com/business/2017/jun/20/uk-gas-storage-prices-rough-british-gas-centrica|access-date=2021-03-02|website=The Guardian|language=en}} This facility, owned by Centrica, was closed in 2017 due to maintenance costs but about 20% of its capacity was reopened in October 2022.{{Cite news |date=2022-10-28 |title=British Gas-owner reopens storage ahead of winter |language=en-GB |work=BBC News |url=https://www.bbc.com/news/business-63423600 |access-date=2023-12-29}}

The operational onshore gas storage facilities in Britain are as follows.{{Cite web|last=Le Fevre|first=Chris|date=2013|title=Gas storage in Great Britain|url=https://www.oxfordenergy.org/wpcms/wp-content/uploads/2013/01/NG-72.pdf|access-date=4 March 2021|website=Oxford Energy}}{{Cite web|title=GB Gas Storage Facilities|url=https://www.ofgem.gov.uk/ofgem-publications/146004|access-date=1 March 2021|website=ofgem}}{{Cite web|title=Storengy UK Stublach site|url=https://www.storengy.co.uk/storengy-uk-stublach-site|access-date=1 March 2021|website=Storengy}}

The salt cavity storage facility at Hornsea, East Yorkshire comprises seven cavities at a depth of {{convert|6000|ft|m|abbr=off}}, which each store up to 60 million m³ (80 million cu. yd.) of gas at a maximum pressure of 240 bar (3500 psi). The releasable volume of gas is about half of the gross volume. During periods of low demand, gas is compressed into the cavities by electrically driven compressors and fed back onto the NTS at times of peak demand.

Salt cavern storage facilities at Stublach were created by drilling {{convert|20|in|cm|abbr=on}} diameter holes through {{convert|1000|ft|m|abbr=off}} of rock and {{convert|820|ft|m|abbr=off}} of salt. This was followed by inserting metal tubes into the holes and filling the annulus with cement to create a leak-tight seal. Water was injected into the wells to dissolve the salt and create brine which was supplied to local industry for the production of bulk chemicals, such as soda ash and chlorine. The caverns created are about {{convert|300|ft|m|abbr=off}} in diameter and {{convert|260|ft|m|abbr=off}} tall (each has a volume of 339,300 m³; 443,800 cu. yd.) and are used to hold compressed gas. Stublach is the largest gas storage facility in the UK, containing up to 450 million cubic metres (590 million cubic yards) of gas.

There are 25 (mostly gas turbine driven) compressor stations and over 25 pressure regulators. Gas moves through the NTS at speeds up to {{convert|25|mph|km/h|abbr=on}}, depending on pressures and pipeline diameters. Compressor stations generally operate at a pressure ratio of 1:1.4 – a balance between maintaining pressure and hence flow, and the capital and running cost of the compressors. It also ensures that the temperature rise across the compressors is not high enough to require after-coolers to prevent damage to the pipeline protective coatings. On the pipelines from St Fergus, compressor stations are provided at {{convert|40|mi|km}} intervals; each compresses the gas from about 48 bar (700 psi) at {{convert|5|°C|°F|abbr=on}} to 65 bar (950 psi) at {{convert|45|°C|°F|abbr=on}}.{{Cite book|title=Gas: Natural Energy|last=Cassidy|first=Richard|publisher=Frederick Muller Limited|year=1979|location=London|pages=46–47}}

Compressor stations include:

• England – Wooler, Bishop Auckland, Carnforth, Nether Kellett, Warrington, Hatton, Alrewas, Wisbech, King's Lynn, Peterborough, Churchover, Huntingdon, Cambridge, Diss, Chelmsford, Aylesbury, Lockerley and Wormington.
• Scotland – Aberdeen, Avonbridge, Kirriemuir, Moffat and St Fergus.

Offtakes from the NTS include those supplying industrial users, local distribution networks, storage sites and export pipelines.

• To about 71 large users such power stations and industry, either on multi-business sites such as Billingham and Runcorn or to individual companies such as INEOS Teesside.
• To inland storage sites (see 'Entry') and formerly to the offshore Rough field storage site via the Easington gas terminal.
• To the Irish interconnectors; the 24-inch {{convert|85|mi|km}} Scotland-Northern Ireland Pipeline (SNIP) to Ballylumford, Northern Ireland, and the two 24-inch UK-Ireland Interconnectors to Dublin, both via an NTS offtake at Moffat, Scotland.
• Gas can be exported to Belgium and the Netherlands via the Interconnector UK and the BBL pipelines, both via the Bacton terminal.
• To the Gas Distribution Network of Local Distribution Zones, as follows:{{Cite web|url=http://www2.nationalgrid.com/WorkArea/DownloadAsset.aspx?id=34164.|title=Gas Transportation Charges Tables 3&5}}

Companies that own part of this gas network, also known as the Local Transmission System (LTS), are known as gas transporters. Gas enters this network via the NTS through a pressure reduction station to the twelve gas distribution zones in Great Britain within eight distribution networks. The network covers {{convert|275000|km|abbr=on}}. The LTS is managed from Hinckley, Leicestershire (former headquarters of the NTS). Financial transactions between gas transporters are managed by Xoserve, based in Solihull, which was a department of National Grid before it became an independent company.

For retail distribution, Cadent owns the network in North West England, the West Midlands, the East Midlands, the East of England and North London. In the North of England, local distribution is owned by Northern Gas Networks; in the Wales and West by Wales and West Utilities; and in Southern England and Scotland by SGN.

The transmission network is owned by National Gas, which is owned by Macquarie Asset Management (80%) and National Grid plc (20%).{{Cite news |last=Bose |first=Kiran |date=2023-01-31 |title=National Gas leaves National Grid |language=en-US |url=https://www.energylivenews.com/2023/01/31/national-gas-leaves-national-grid/ |access-date=2023-01-31}} Agreement was reached in July 2024 for Macquarie to buy National Grid's stake, giving Macquarie full ownership in the first quarter of 2025 subject to regulatory approval.{{Cite web |last= |first= |date=26 July 2024 |title=Macquarie to acquire remaining stake in UK's National Gas |url=https://www.reuters.com/markets/deals/macquarie-acquire-remaining-stake-uks-national-gas-2024-07-26/ |access-date=18 September 2024 |website=Reuters}}

The changing ownership of the NTS reflects developments and corporate changes in the UK's gas and energy industries.

• Gas Council and area boards, 1962 – 31 December 1972
• British Gas Corporation, 1 January 1973 – 24 August 1986
• British Gas plc, 24 August 1986 – 1994
• Transco plc, part of British Gas plc, 1994 – 17 February 1997
• Transco plc, part of BG plc, 17 February 1997 – 1999
• Transco plc, part of BG Group plc, 1999 – 23 October 2000
• Transco plc, part of Lattice Group plc, 23 October 2000 – 21 October 2002
• Transco plc, part of National Grid Transco plc, 21 October 2002 – 31 January 2023; renamed National Grid Gas plc on 10 October 2005
• National Grid Gas plc, owned by Macquarie, BCI and National Grid, from 31 January 2023; renamed National Gas Transmission plc on 6 February 2023

Northern Ireland is not part of the NTS and gets its gas via the Scotland-Northern Ireland pipeline (SNIP), owned by Premier Transmission and built between 1994 and 1996. The gas network in Northern Ireland is split, with one area owned by Phoenix Natural Gas and the other by Firmus Energy.

• Humber Gas Tunnel
• Central Area Transmission System
• United Kingdom–Ireland natural gas interconnectors

{{reflist|30em}}

• [http://www2.nationalgrid.com/uk/industry-information/gas-transmission-system-operations/ Gas Transmission System Operations]
• [https://www.nationalgridgas.com/land-and-assets/network-route-maps Map of the NTS at National Grid]
• [http://www.see.ed.ac.uk/~mzaiser/4thyear/websites05/MacRonald/Website/3-2.html Detailed map of the NTS]
• [http://www2.nationalgrid.com/UK/Industry-information/Gas-transmission-operational-data/Instantaneous-flows/ Instantaneous gas flows into the NTS]
• [http://www.nationalgrid.com/uk/Gas/Pipelines/ Pipeline projects at National Grid plc]
• [https://web.archive.org/web/20080213110828/http://www.nationalgrid.com/uk/Gas/About/How+Gas+is+Delivered/ Ownership of the gas distribution network], archived in 2008
• [https://archive.today/20121224083155/http://www.sbgi.org.uk/utilitynetworks Society of British Gas Industries], archived in 2012
• [http://www.xoserve.com Xoserve]
• [https://web.archive.org/web/20110716174716/http://www.energynetworks.org/education/GCSE_factsheets/GasFacts03_080119.pdf GCSE PDF factsheet about the NTS], archived from www.energynetworks.org in 2011
• [http://www.oldflames.org.uk/Plant%20Operations%20Department.pdf Origins and Growth of the British Gas Plant Operations Department (including pipeline maps)] www.oldflames.org.uk
• [http://www2.nationalgrid.com/WorkArea/DownloadAsset.aspx?id=34164. Statement of Gas Transmission Transportation Charges Table 3 & 5]

{{Energy in the United Kingdom|sources}}

Category:Natural gas pipelines in the United Kingdom